STRESS PERTURBATIONS ASSOCIATED WITH ACTIVE FAULTS PENETRATED BY BOREHOLES - POSSIBLE EVIDENCE FOR NEAR-COMPLETE STRESS DROP AND A NEW TECHNIQUE FOR STRESS MAGNITUDE MEASUREMENT

Detailed studies of stress-induced wellbore breakouts in wells drilled through active faults reveal stress field discontinuities that are apparently associated with recent fault movements. These discontinuities are expressed as localized rotations in wellbore breakout orientation in the vicinity of the fault penetrated by the borehole. This phenomenon is observed in a variety of tectonic environments and rock types. Utilizing cases where relatively complete knowledge of the horizontal principal stresses is available from in situ measurements, we use three-dimensional dislocation modeling to demonstrate that these discontinuities can be explained as the superposition of a reference stress state and a perturbation caused by movement on preexisting faults. Case studies from normal, strike-slip and reverse faulting stress states indicate that nearly complete stress drop is required to match the observed breakout orientation anomalies. Hydraulic fracturing data independently confirm the occurrence of near-complete stress drop on some faults penetrated by drilling. Modeling of the observed interactions between breakouts and fractures can also be used to obtain information about the magnitude of in situ stress.